Rare-earth-doped optical fibre amplifiers, particularly the erbium-doped fibre, have recently attracted very considerable attention in the field of optical fibre communications. See, for example, "Low noise erbium-doped fibre amplifier operating at 1.54 .mu.m", Electronics Letters, Vol. 23, No. 19, p. 1026, 1987 and United Kingdom patent application 2180392. The erbium-doped fibre amplifier (EDFA) conveniently operates in the preferred telecommunications spectral-window located at a wavelength of 1.55 .mu.m. In addition, it has been shown to have high polarisation-insensitive gain (greater than 30 dB), low crosstalk between signals at different wavelengths, good saturation output power (&gt;1 mW) and a noise figure close to the fundamental quantum limit (approximately 3 dB). The excellent noise characteristics potentially allow hundreds of amplifiers to be incorporated along the length of a fibre telecommunication link, which could then span more than 10,000 km. Compared to the alternative of a transmission link with electronic repeaters, an all optical link has the merit that it is transparent to the transmission-code format and bit-rate. It can thus be uprated by changing only the transmitter and receiver, and not the repeaters.
Despite its generally-excellent characteristics, the erbium-doped-fibre amplifier has one major disadvantage, namely its spectral linewidth. The 1.55 .mu.m telecommunications window is approximately 20 nm wide and an ideal amplifier would have a flat spectral-gain across the full window. Although the broad fluorescence linewidth of ions in glass provides wideband amplification, their spectral-gain characteristics are often irregular. For example, the fluorescence band of erbium-doped fibre amplifiers (EDFA) is due to the radiative transition between the .sup.4 I.sub.13/2 to .sup.4 I.sub.15/2 energy levels. Both of these levels are broadened and Stark-split to produce a manifold of contributing levels. Transitions between the .sup.4 I.sub.13/2 manifold and the .sup.4 I.sub.15/2 manifold are not equally likely and, in particular, the transition between the two levels corresponding to the peak gain wavelength has the highest probability. This wavelength varies from 1530 nm to 1535 nm, depending on the host glass material. It is therefore well-known that the spectral gain of an EDFA has a peak response as shown in FIG. 1. It can be seen that because of the irregular shape of the spectral gain, the amplifier has a 3 dB gain-bandwidth of only 4.5 nm.
If it is intended to use the amplifier in a telecommunications system which employs a single signal-wavelength corresponding to the peak gain of the erbium amplifier (1535 nm in the case of a germanosilicate host glass), the narrow spectral gain is no disadvantage and may indeed be an advantage. However, if the telecommunications link is required to operate at a number of optical wavelengths and to exploit fully the available low-loss window offered by current telecommunications fibres, the large variation in gain across the spectrum can cause problems. Referring to FIG. 1 (plotted for an alumino-silicate host glass), it is clear that operating between wavelengths of 1540 nm and 1560 nm offers a broad gain-plateau with a gain reduced by some 8 dB relative to that obtainable at the peak. It is perfectly possible to operate in this reduced-gain plateau, see R. Welter, R. I. Laming, R. S. Vodhanel, W. B. Sessa, M. W. Maeda and R. E. Wagner, "Performance of an erbium-doped fibre amplifier in a 16-channel coherent broadcast network experiment" Proc. CLEO, Paper PD22, Baltimore 1989. However, the presence of an adjacent high-gain region at 1531 nm presents a number of disadvantages, as follows:
1. Laser Oscillation--A high-gain optical amplifier tends to oscillate owing to the existence of feedback from unavoidable reflections and backscatter from the fibre transmission link. Typically, the gain should be limited to around 30 dB in practical applications. Thus, in an amplifier having a peaked response, the maximum gain at the peak must be limited to 30 dB to avoid oscillation, thus giving a gain elsewhere in the spectrum of little over 20 dB.
2. Poor Pumping Efficiency--In a high-gain optical amplifier operating in the small-signal regime, the pump power required to achieve a given gain is dominated by the build up of unwanted amplified-spontaneous-emission (ASE). Spontaneous emission occurs throughout the length of the amplifier but, in particular, only that which originates at the input of the amplifier experiences the full gain through the amplifier. Thus spontaneous emission when subject to a high gain contributes a substantial light level at the amplifier output and can saturate the amplifier output section. Under these ASE-induced saturation conditions, the pump efficiency (i.e. gain/pump power) rapidly decreases and much of the available pump power is converted to ASE at the output of the amplifier, rather than contributing to the gain of the signal. The amount of ASE generated is very nearly proportional to the amplifier gain. Thus the situation is worsened if a high-gain spectral region exists adjacent to that in which it is wished to operate. Taking the example of the gain spectrum shown in FIG. 1, to obtain a 24 dB gain at 1550 nm we cannot avoid having a gain of 32 dB at 1531 nm and must accept the large value of ASE at this wavelength. Thus a substantial amount of the pump power is wasted in supplying ASE power at the gain peak of 1531 nm and this leads to poor pump efficiency. Put another way, to obtain a 24 dB amplifier, we must pump at a rate appropriate to a 32 dB amplifier.
3. Increased Spontaneous-Spontaneous Beat Noise--Apart from reduced pump efficiency, the presence of a large level of ASE at an adjacent wavelength to the signal will contribute a higher level of spontaneous-spontaneous beat noise. This will degrade the amplifier noise figure, particularly under small-signal input conditions.
4. Saturation Problems--For wavelength-division multiplexed signals, difficulties will be experienced in using an amplifier with widely-differing gain for each of the different signal wavelength-channels. A danger exists of the high-gain channel at 1531 nm saturating the amplifier and thus reducing the gain for all the other channels. Saturation can also lead to increased interchannel crosstalk.
All of the above problems would be alleviated if the amplifier had a perfectly-flat spectral gain. The local environment for the erbium ion has a considerable effect on its gain spectrum and it is well known that an alumino-silicate host glass provides a broader gain spectrum. Previous work has further smoothed this spectrum by use of a pump wavelength of 1.48 .mu.m and careful choice of pump power, C. A. Atkins, J. F. Massicott, J. R. Armitage, R. Wyatt, B. J. Ainslie and S. P. Craig-Ryan, "High-gain broad spectral bandwidth erbium-doped fibre amplifier pumped near 1.5 .mu.m", Electronics Letters, Vol. 25, pp. 910-911, 1989. However, this was achieved at the expense of a lower value of population inversion, reduced pump efficiency and a higher noise figure.